Lattice modifier deforms object as soon as an object is selected
Changes to the lattice while in edit mode will be used to deform the mesh. This applies even if you alter the lattice before linking with the modifier.

Also selecting and scaling all points in edit mode, like you would with a mesh object, will add an odd out of proportion scaling effect while scaling the lattice in object mode.

To prevent this you need to scale the lattice in object mode to get it to the starting position that you want before making any changes in edit mode.